Related על מגילה 1:1

Phylacteries and Mezuzoth may only be written in square Hebrew script. Permission was also given to write scrolls of the Law in Greek characters but not in those of any other foreign tongue. The ancient Greek script has gone out of use, been corrupted and become obsolete. Hence, at the present day, all three religious articles,—Scroll of the Law, Tephillin and Mezuzah,—are only written in the square Hebrew script. Care must be taken in writing that no letter shall adhere to any other letter; for any letter which is not completely surrounded on all four sides by a blank space of parchment has been improperly written. Any letter that cannot be read by a child of ordinary intelligence—not particularly bright or dull—is improperly written. Thus a Yod must not be like a Vav nor a Vav like a Yod, nor a Koph like a Beth, nor a Beth like a Koph, nor a Daleth like a Resh, nor a Resh like a Daleth, etc., so that the reader may be able to read fluently.